Conductive material and process for preparing the same

ABSTRACT

The invention provides a conductor comprising a reaction-sintered body of a conductive nitride produced from a powder of at least one metal selected from Ti, Zr, V, Nb, Ta, Cr, Ce, Co, Mn, Hf, W, Mo, Fe, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, Lu, Th and Ni, and a process for producing such conductor by heating a molding containing a metal powder in a nitriding gaseous atmosphere containing no CO gas.

This is a division of application Ser. No. 07/319,307, filed Mar. 6,1989 now U.S. Pat. No. 5,085,806.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a conductive material comprising areaction-sintered body which incurs only a slight dimensional change inthe sintering operation, and a process for producing such conductivematerial

2. Description of Related Art

Highly heat resistant SiC, Si₃ N₄ and the like are generally known asengineering ceramics suited for use as structural material for engines,turbines and the like. Among the known techniques for sintering suchmaterials are the so-called pressureless sintering method, pressuresintering method and reaction-sintering method. Among them, apressureless sintering method and pressure sintering method carry a highrisk of deformation and are poor in dimensional precision, with thepossible rate of dimensional change at the time of sintering running ashigh as 15 to 20%, and highgrade techniques are required for thesemethods on the part of the workers. The high rate of dimensional changeduring sintering requires much time and cost for post-sintering work,and this has been one of the greatest obstructions to the popular use ofengineering ceramics. As for the reaction-sintering method, on the otherhand, it is known that the rate of dimensional change suffered at thetime of sintering is small in comparison with the materials sintered byother methods, and it is only disclosed in Japanese Patent Kokai(Laid-Open) No. 58-140375 that the material is composed of a nitride ofmetallic Si powder and little is known about the conductivity of suchmaterial.

Si₃ N₄ binding material, which has been generally used as a refractory,is also a material which is expected to suffer little dimensional changein sintering. Such material is disclosed in Japanese Patent Kokai(Laid-Open) No. 58-88169, but nothing is told about conductivity.Further, the mechanical strength of this material is as low as about 50MPa, which frustrates any expectation of its use as a structuralmaterial.

Lately, request is strong for the development of conductive ceramicsuseful as heat-resistant heater materials or conductors. Invention ofceramics with small electrical resistivity would contribute greatly tothe improvement of performance of currently used products and would alsopave the way for new and wider use of ceramics. In the conventionalconductive ceramics, as for instance disclosed in Japanese Patent Kokai(Laid-Open) Nos. 50-84936, and 60-44990, a conductive compound is mixedwith SiC or Si₃ N₄ and the mixture is hot-press sintered to solve theproblem of electroconductivity. The hot press method, however, involvesthe problem of high production costs as it requires a vast amount ofenergy for sintering.

Further, Japanese Patent Kokai (Laid-Open) No. 60-60983 discloses amethod for obtaining a conductive ceramic material by mixing aconductive compound with Si₃ N₄ and subjecting the mixture topressureless sintering which is more advantageous in terms of energythan the hot press method. However, since this method utilizes asintering aid, the rate of dimensional change at the time of sinteringmay rise up to 15-18%, posing the problem of intolerable deformation.

Japanese Patent Kokai (Laid-Open) No. 61-247662 discloses a sinteredbody comprising a Cr carbonitride obtained by reacting and sintering aCr powder molding with CO in N₂ gas at 1,500° C., such sintered bodybeing described as having a specific resistance at room temperature ofmore than 10⁻⁴ Ωcm. In this case, since a Cr powder molding is reactedand sintered in a CO-containing N₂ gas, the produced sintered bodydiffers in composition between the surface portion and the insideportion, that is, the obtained sintered body has different properties atits surface and in the inside, and thus there can not be obtained asintered body with uniform quality.

As described above, there have been available no practical techniquesfor producing ceramics having excellent dimensional precision andcapable of controlling electrical resistivity to a low level.

The aforementioned conventional techniques have been deficient in therate of dimensional change at the time of sintering, electricalresistivity, mechanical strength, etc., and the use of the products asmechanical structural material or functional material has been limited.

SUMMARY OF THE INVENTION

The object of the present invention is to provide conductor made of areaction-sintered body which suffers little dimensional change in thesintering operation and has a uniform composition, a process forproducing such conductor and uses thereof.

The conductor according to the present invention is composed of areaction-sintered body of a nitride produced from a metallic powder, thesintered body being minimized in dimensional change in the sinteringoperation by effecting interparticle bonding while reducing the voidsbetween particles by a conductive nitride produced from a metallicpowder in the molding during sintering. The present invention isintended to enable optional control of electrical resistivity accordingto the type of reaction-sintered body.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, FIG. 3 and FIG. 6 are the graphs showing the relation betweenelectrical resistivity and material blending ratio.

FIG. 2 and FIG. 4 are the graphs showing the relation between bendingstrength and material blending ratio.

FIG. 5 is a graph showing the relation between porosity of sintered bodyand bending strength.

FIG. 7 is a graph showing the relation between amount of organic binderadded and bending strength.

FIG. 8 and FIG. 9 are a sectional view and a side elevational view,respectively, of a collector ring for an AC genenator for automobiles.

FIG. 10(a) and FIG. 10(b) are a sectional view and a side elevationalview, respectively, of a commutator.

FIG. 11(a) and FIG. 11(b) are a plan view and a side view, respectively,of a heater.

FIG. 12 is a graph showing the relation among heater electrificationtime, heater temperature, terminal voltage and load current.

FIG. 13 is a schematical cross-sectional view illustrating an automobilestarter using the conductive material according to this invention.

FIG. 14 is a schematical cross-sectional view illustrating an alternatorfor automobiles using the conductive material of this invention.

FIG. 15 is a schematical view showing the geometry of a slip ring madeby using the conductive material of this invention.

FIG. 16(a) is an axial sectional view of the slip ring.

FIG. 16(b) is a pictorial view of a brush.

DETAILED DESCRIPTION OF THE INVENTION

The metallic powder used in the present invention is a powder of a metalor metals belonging to Group III to Group VIII of the Periodic Table,and it comprises at least one metal selected from Ti, Zr, V, Nb, Ta, Cr,Ce, Co, Mn, Hf, W, Mo, Fe, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, Lu, Thand Ni. For instance, a nitride of Ti can provide a sintered body of lowresistivity since the electrical resistivity of such nitride is as lowas about 5×10⁻⁵ Ωcm or less. Also, mixing of at least one of powdered Siand Al with said metallic powder can greatly change resistivity sincethe nitride of Si or Al has as high an electrical resistivity as theorder of 10¹⁵ Ωcm. Thus, the resistivity of the nitride varies dependingon the type of metallic powder used, and by properly combining suchmetallic nitrides of different resistivities, it becomes possible tocontrol resistivity over a wide range from conductor to insulator.Control of resistivity is also made possible by mixing a metallic powderand an inorganic compound.

The metal nitride obtained has reaction product particles or whiskers of30 μm or less. This product is formed in a net shape relative to theother unreacted product and combines with it.

In the present invention, a recommended means for controllingresistivity is to reduce the content of at least one of silicon nitrideand aluminum nitride formed from Si and Al powder to 90% by volume orless.

In the present invention, it is desirable to keep the content ofinorganic compound at or below 7% by volume for preventing a drop ofstrength.

The inorganic compound used in the present invention should preferablyhave an average particle size of 100 μm or less for maintaining filmstrength. Whiskers or long fibers may be used as the inorganic compound.The whiskers used in the present invention are preferably ones having anaverage aspect ratio of 200 or less and an average length of 200 μm orless. Use of long fibers has the advantage of making it possible to givean orientation to the electrical properties by properly orienting suchfibers.

In the present invention, it is preferred to adjust the porosity to 30%or less for preventing a reduction of strength. In reaction-sinteringunder atmospheric pressure, the porosity becomes 5% or more in thepractical production processes, but when sintering is conducted under apressured atmosphere, the porosity becomes 1% or less. The porescomprise open pores. This is for the following reason: for obtaining asintered body by sintering a molding made of metallic powder under anitriding gas atmosphere and changing such metallic powder into anitride through reaction with said nitriding gas, presence of vent holesis necessary for allowing the nitriding gas to pass through the moldingIn this sintering operation, there may occur a reaction between thepowder metals in case the powder metals used are of specific types, butsuch reaction presents no problem relating to the properties of thesintered body. As a result of such reaction, there may be partlyproduced TiAl, TiAl₃, TiSi, ZrAl or the like.

In the present invention, further reduction of porosity is possible bysubjecting the obtained sintered body to such treatment as hot pressing,hot isostatic pressing or post-sintering. Such treatments, however, areundesirable where a high dimensional precision is required.

The molding is heated for a long time under a nitriding gas atmosphereconsisting of nitrogen and/or ammonia, and if necessary with hydrogen,argon, helium or the like added thereto, at a temperature below themelting point of metallic powder in the initial stage of sintering, andthen further heated preferably at a high temperature, such as 600° to1,350° C., especially 1,100° to 1,350° C. No CO gas is substantiallycontained.

In the present invention, the molding binder is prepared by adding anorganic high molecular compound such as butyral or polyethylene and anorganic Si high molecular compound such as siliconimide compound orpolysilane compound, each in a suitable amount, preferably 8 to 15 partsby weight, and the particle packing density in the molding is adjustedpreferably to 65% or more. The reason for this is described below.

The starting powder itself is composed of brittle solid particles, sothat when the powder is simply pressed, it is difficult to effectdesired packing and the molding may develop strain or even cracks as thepressure is increased. It is therefore necessary to add a certainorganic binder to assist fluidity of the powder and to increase thestrength of the molding. The strength of the sintered body is affectedby the amount of the organic binder added. Such change of strength ofthe sintered body according to the amount of the organic binder added isapparent, for example, from the relation between the amount of organicbinder added and the strength of TiN sintered body obtained according tothe sintering method of the present invention by using metallic Tipowder. This phenomenon has a relation to the particle volume packingdensity in the molding. The particle volume packing density in themolding will not increase when the amount of the organic binder is toolarge or too small, but when the optimal amount to be added isincreased, the fluidity of the mixture under heating is improved tofacilitate pressure molding, resulting in an improved particle volumepacking density in the molding. However, if the organic binder is mixedmore than void rate when the starting powder is in an ideal closelypacked state, the starting powder is rendered into an isolated state inthe binder. Good fluidity of the mixture is maintained, but the solidcontent in the molding decreases as the binder increases, resulting in areduction of particle volume packing density in the molding.

In the present invention, the cells in the composite ceramic sinteredbody may be impregnated with particles of a resin, oil or the like.Applications of the present invention to sliding members are alsopossible.

The electrically conductive ceramics according to the present inventioncan be subjected to discharging works by reducing electricalresistivity.

In the present invention, it is preferred that the compound producedfrom the metallic powder would form the center of the nitride phase,because when sintering is carried out in an oxidizing gas, an oxidephase is formed, making it difficult to control electrical resistivity.

According to the present invention, by carrying out thereaction-sintering operations with nitrides produced from various metalpowders, there can be obtained uniform ceramics minimized in dimensionalchange in the sintering operation and controllable in electricalresistivity optionally from conduction to insulation.

A prominent feature of the present invention resides in interparticlelinkage of inorganic compounds by the particles and whiskers ofconductive nitrides produced from said metal powders. This can beattained by adding a binder made of a thermoplastic resin in said metalpowders, kneading the mixture under heating, subjecting the mixture tohot pressure molding to form a molding with a particle volume packingdensity of 60 vol % or more, removing the binder in said molding by aheat treatment, and then sintering the molding by heating in a nitridinggas atmosphere free of CO gas.

In the above process, a binder is added in an amount defined as follows:

    B=[(7S/20,000)+3]±2.5

wherein B is the amount of binder added, expressed by parts by weight to100 parts by weight of the starting powder composition, and S isspecific surface area (cm² /g) of starting powder. After kneading underheating, the mixture is subjected to hot press molding to form a moldingwith particle volume packing density of preferably 70 vol % or more.

Thus, preferably the composition contains a binder composed of saidmixed powder and thermoplastic resin and has an apparent viscosity at150° C. of 3 to 9×10⁴ N·s/m², the amount of binder being determined asdescribed above.

The particle diameter of said mixed metal powder should be not more than10 μm, preferably not more than 1 μm, and the particle diameter of thepowder of inorganic compound should be not more than 100 μm, preferablynot more than 20 μm. As for these powders, the commercially availableproducts can be used in the form as they are, but it is recommended togrind them by a mill or other means to form spherical particles for usein this invention.

Said inorganic compound powder may be partly replaced with whiskers. Theamount of whiskers used is preferably not more than 55 vol % in thesintered body composition. Use of a greater amount of whiskers may makeuniform blending of the starting materials hard. The whiskers used inthis invention are preferably those having an average aspect ratio of200 or less and an average length of 200 μm or less.

As binder, it is possible to use a thermoplastic resin employed for thepreparation of preforms for ceramic sintering. For example,polyvinylbutyral, polyethylene, polysilicone, synthetic waxes and thelike are usable.

Such binder plays an important role in this invention. That is, theamount of binder added proves to be a decisive factor for adjusting theparticle volume packing density in the molding to less than 60 vol %.

The present inventors have studied further on this finding and disclosedthat there existed a very profound interrelation between the specificsurface area of starting powder and the amount of binder added ascalculated per 100 parts by weight of the starting powder, and thepresent invention has been reached from these findings.

First, the specific surface area S (cm² /g) of starting powder can bedetermined as follows: ##EQU1## wherein σ is a density and d is anaverage particle size (μm). The relation between said specific surfacearea S and binder amount B making the particle packing density in themolding greater than 60 vol % (the amount being parts by weight to 100parts by weight of starting powder composition) is given by the formulaalready shown before (B=[(7S/20,000)+3]±2.5).

By adding binder in an amount within the range defined by the formulaabove, it is possible to make the particle packing density in themolding higher than 70 vol % and to provide a sintered body having aflexural strength of about 300 MN/m² or higher

The starting composition blended with said specified amount of binderhas its apparent viscosity defined in the range of 3 to 90×10⁴ N·s/m²,and by selecting this range of viscosity, it is possible to predictfluidity at the time of molding and to obtain a molding with a packingdensity higher than 60 vol %, so that a composition suited for near-netshaping can be provided. Especially by making a packing density higherthan 70 vol %, there can be obtained a composition with even higherstrength. In practical use, a packing density not higher than 85 vol %is preferred, and near-net shape moldings are obtainable.

From the viewpoint of fluidity of the starting composition, use of ametal powder with a particle size not greater than 1 μm is preferred. Asbinder, it is advised to add a blend comprising 15 to 60% by weight ofpolyethylene, 30 to 70% by weight of wax and 5 to 25% by weight ofstearic acid.

The starting composition with the binder resin added thereto issufficiently kneaded and then molded. Molding can be accomplished byusing various molding methods such as injection molding, press molding,rubber press molding, extrusion molding, doctor blade method, metallicpowder molding, etc., from which a proper method is selected accordingto the shape of the molded product, required properties and otherfactors. Usually, hot molding is carried out at a temperature above thesoftening point of the binder resin. When molding is conducted by usinga mechanical press, a molding pressure of about 1,000 kgf/cm² isapplied. A doctor blade method is recommended for manufacturing thinsubstrates.

The molding is degreased (cleared of binder) before sintered. Degreasingcan be accomplished by heating the molding slowly from room temperatureat a rate of about 2° C./hr until reaching about 500° C.

A heating rate of 4° C./hr till reaching the sintering temperature issuited for effecting desired sintering with ease. If necessary, hotpress may be applied.

The sintered body obtained according to this invention preferably has aporosity below 30%. When the porosity exceeds 30%, the sintered body isreduced in strength. A porosity below 30% can be accomplished byadjusting the particle volume packing density in said molding to greaterthan 60 vol %.

In the sintered body are present whiskers of nitrides produced in thesintering process. It is desirable that such whiskers be contained in anamount of 1 to 70 vol %, especially 10 to 30 vol %, based on thereaction product phase.

The notably small dimensional change (about 0.15% or less) in thesintering operation in the production of ceramics according to thepresent invention may be accounted for as follows.

First, the dimensional change in the sintering operation is highlyassociated with the nitride whiskers produced by sintering in anitriding atmosphere. It is desirable that such whiskers be adjusted tobe present in an amount of 1 to 30 vol % in the nitride produced.

Regarding the relation between blending ratio of starting materials(metal/(metal+inorganic compound)) and dimensional change at the time ofsintering and flexural strength in a sintering operation carried out byadding 9 parts by weight of a thermoplastic resin to a mixture of metalpowder and inorganic compound, mixing them under heating, subjecting themixture to hot press molding and, after removing the binder, sinteringthe molding in a nitrogen gas, the fact is noted that whiskers of theproduced nitride increase and the strength rises as the amount of metalused increases. Dimensional change also increases during sintering, butit is practically insignificant.

This is considered attributable to close interparticle linkage of thesintered body by the whiskers produced in the sintering operation.Especially, obtainment of a sintered body with as high a flexuralstrength as 300 MN/m² or more by the presence of 45 vol % or more ofwhiskers is considered ascribed to such an increase of linkage chains.

Flexural strength is greatly influenced by the amount of binder resinused. This is associated with the volume packing density of particles inthe molding.

Starting powder itself comprises brittle solid fine particles, andpacking thereof is difficult by simple pressing Therefore, it isnecessary to add a binder to enhance fluidity of the powder and to alsoelevate strength of the molding. The strength of sintered body ischanged according to the amount of binder added. As already mentioned,this is associated with the volume packing density of particles in themolding. Increase of binder improves fluidity of the heated mixture tofacilitate pressure molding. As a result, volume packing density ofparticles in the molding increases. However, when the binder is added ina larger amount than the void volume in a case such that the startingpowder is in a state of ideal closed packing, the starting powder isrendered into an isolated state in the binder, and although fluidity isimproved, the solid content in the molding decreases to cause acorresponding reduction of volume packing density of particles in themolding, resulting in an increased porosity and reduced strength of thesintered body.

As described above, the whiskers produced from metal powder sintered byheating in a nitriding atmosphere function for effecting interparticlelinkage while also serving for filling the voids between the particlesto make a three-dimensional growth in the sintered body, making itpossible to obtain ceramics with high toughness.

In accordance with the present invention, it is possible to easilyobtain a ceramic material which suffers not more than 2% of dimensionalchange when sintered and whose electrical resistivity can be controlledoptionally from a conductive state to an insulative state. The presentinvention also makes it possible to easily obtain a ceramic complexwhich is small in dimensional change by sintering and can have a desiredresistivity within the range from 10¹⁴ to 10⁵ Ωcm by adjusting theblended amounts of an electrically conductive compound and insulatingcompound. Since the present invention cannot substantially require anyworking costs after sintering, the present invention can expand thescope of use of ceramics to a variety of fields including not onlystructural parts such as engine and turbines but also to various typesof heaters, electrode materials, motor brush, commutator, substrates,current collector, aircraft and space technology, iron and steel,oceanographic development, etc.

EXAMPLE 1

To a metallic Ti powder having an average particle size of 1 μm wasadded 4 parts by weight of an organic binder comprising polyethylenewax, synthetic wax and stearic acid as molding binder, and the mixturewas kneaded by a press kneader under heating at 160° C., at which theresin is softened, for a period of 5 hours. After cooling, the kneadedmixture was crushed to prepare a sample starting material. This materialwas mechanical press molded under a molding pressure of 1,000 kgf/cm² ata temperature of 160° C. to form a molding of 50 mm in diameter and 20mm in thickness. The volume packing density of particles in the obtainedmolding was greater than 60 vol %. This molding was heated in an argongas atmosphere at a heating rate of 3° C./hr to 500° C., and then afterremoving the molding binder, the molding was further heated in anitrogen gas atmosphere at temperatures of about 10 stages from 600° C.to 1,300° C., heating at each stage being conducted for an almost equaltime, thus heating the molding for a total period of about 80 hours atrespective temperatures to obtain a sintered body. The properties of thethus obtained sintered body are shown in Table 1.

By way of comparison, a sintered body was obtained by using a startingmaterial consisting of 40 vol % of Si₃ N₄ powder having an averageparticle size of 0.8 μm, 55 vol % of TiN powder and 5 vol % of Y₂ O₃ asa sintering aid, molding the mixture in the same way as described above,and sintering the molding at 1,700° C. for 5 hours. The properties ofthis sintered body are also shown in Table 1.

It will be seen that in the product of the present invention theparticles are bound with TiN alone without intermediation of othersubstances at grain boundaries, and as compared with the comparativesintered body in which a sintering aid Y₂ O₃ was added to Si₃ N₄ powderand TiN powder for binding the particles with additional Y₂ O₃ at grainboundaries, the product of the present invention is very small (0.8%) indimensional change during sintering. It is thus possible according tothe present invention to obtain ceramics having good near-net shapingcharacteristics and low electrical resistivity. Dimensional changeindicates the change of length. It was also confirmed that a slightamount of metal remained in the sintered body of the present invention.Electrical resistivity shown here is measured along a section of the cutsintered body. Resistivity from the surface was also measured bychanging the cut section, but resistivity was substantially the same forboth the inside and outside.

The sintered body of the present invention formed therein approximately2 vol % of whiskers. A three-point bending test according to the JIS wasused for the flexural test in the present invention.

                  TABLE 1                                                         ______________________________________                                               Dimensional change                                                                         Electrical Flexural                                              during sintering                                                                           resistivity                                                                              strength                                              (%)          (Ω cm)                                                                             (MPa)                                          ______________________________________                                        Example 1                                                                               0.8           3 × 10.sup.-5                                                                      360                                        Comp.    13.4           1 × 10.sup.-4                                                                      340                                        Example 1                                                                     ______________________________________                                    

EXAMPLE 2-23

Molding and sintering were carried out by following the same procedureasExample 1, except that the metals (powder) shown in Table 2 were used inplace of Ti. The properties of the obtained sintered bodies are shown inTable 2. It is noted that the products of the present invention are all1% or less in dimensional change in the sintering operation and have ahigh flexural strength (300 MPa or above). Also, their electricalresistivity can be optionally controlled in the range from 10⁴ to 10⁻⁵Ωcm according to the type and amount of metal powder used. In theproducts of these Examples, too, about 2 vol % of whiskers were formedand a slight amount of metal(s) remained.

                                      TABLE 2                                     __________________________________________________________________________            Ratio of material(s)                                                                              Electrical                                                                          Flexural                                            (powder) blended                                                                        Dimensional change                                                                      resistivity                                                                         strength                                    EXAMPLE (wt %)    in sintering (%)                                                                        (Ω cm)                                                                        (MPa)                                       __________________________________________________________________________    2       Zr:100    0.8       2 × 10.sup.-5                                                                 345                                         3       V:100     1.0       8 × 10.sup.-5                                                                 320                                         4       Nb:100    0.9       1 × 10.sup.-4                                                                 300                                         5       Ta:100    0.7       2 × 10.sup.-4                                                                 312                                         6       Cr:100    0.8       9 × 10.sup.-5                                                                 305                                         7       Ce:100    1.0       2 × 10.sup.-5                                                                 345                                         8       Ti:80, Si:20                                                                            0.5       6 × 10.sup.-5                                                                 365                                         9       Ti:70, Al:30                                                                            0.7       2 × 10.sup.-4                                                                 348                                         10      Ti:20, Si:80                                                                            0.6       5 × 10.sup.-1                                                                 370                                         11      Ti:10, Si:90                                                                            0.6       4 × 10.sup.-3                                                                 372                                         12      Ti:90, Co:10                                                                            0.8       5 × 10.sup.-5                                                                 338                                         13      Ti:90, W:10                                                                             0.7       6 × 10.sup.-5                                                                 340                                         14      Zr:85, Fe:15                                                                            0.7       4 × 10.sup.-4                                                                 324                                         15      Zr:85, Pr:15                                                                            0.9       7 × 10.sup.-4                                                                 305                                         16      Zr:85, Yb:15                                                                            0.9       6 × 10.sup.-4                                                                 302                                         17      Cr:95, Gd:5                                                                             0.8       9 × 10.sup.-4                                                                 325                                         18      Zr:85, Ho:15                                                                            0.8       4 × 10.sup.-4                                                                 335                                         19      Ta:85, Sm:15                                                                            0.8       8 × 10.sup.-4                                                                 298                                         20      Zr:85, Nd:15                                                                            0.8       4 × 10.sup.-4                                                                 312                                         21      Zr:85, Dy:15                                                                            0.8       9 × 10.sup.-4                                                                 305                                         22      Ti:83, Eu:9, Lu:8                                                                       0.8       9 × 10.sup.-5                                                                 352                                         23      Zr:80, Ni:15, Th:5                                                                      0.8       8 × 10.sup.-5                                                                 345                                         __________________________________________________________________________

EXAMPLE 24

A sintered body was obtained by carrying out molding and reactionsintering in the same manner as Example 1, except for changing theblending ratio of metallic Ti powder having an average particle size of1 μm and metallic Si powder having an average particle size of 1 μm. Thecomposition of the obtained sintered body was a 2-phase mixture of TiNand Si₃ N₄, and in this sintered body, about 2 vol % of whiskers wereformed and a slight amount of metals remained.

In FIG. 1 is shown the relation between electrical resistivity and ratioof material blended, and FIG. 2 shows the result of a test conducted onthe relation between flexural strength and material ratio. It is seenfrom these graphs that electrical resistivity is variable within therange of 10⁴ to 10⁻⁵ Ωcm by changing the ratio of material blended It isalso learned that flexural strength is 300 MPa or above irrespective ofthe ratio of material blended.

EXAMPLE 25

A sintered body was obtained by repeating the same molding andreaction-sintering operations as in Example 1, except for changing theblending ratio of metallic Ti powder having an average particle size of1 μm and that of metallic Al powder having an average particle size of 1μm. The composition of the obtained sintered body was a 2-phase mixtureof TiN and AlN, in which whiskers were formed and a small quantity ofmetal remained.

The results of tests on the relation between electrical resistivity andblending ratio of material and on the relation between flexural strengthand blending ratio of material are shown in FIG. 3 and FIG. 4,respectively. From these test results, it is seen that electricalresistivity is variable within the range of 10.sup. to 10⁻⁵ Ωcmdepending on blending ratio of material, and that flexural strength ofthe obtained sintered body is not lower than 250 MPa regardless ofblending ratio.

EXAMPLES 26-30

Sintered bodies with different porosities were obtained by changing theamount of molding binder used in otherwise the same way as Example 1,and these sintered bodies were subjected to the same tests as describedabove. The results are shown in Table 3. The relation between porosityand flexural strength is shown in FIG. 5.

It is known from these results that a flexural strength of 250 MPa orabove can be obtained when the porosity is 30% or less, but the flexuralstrength lowers as the porosity increases, and when the porosity exceeds30%, the flexural strength sharply drops to less than 250 MPa.

                  TABLE 3                                                         ______________________________________                                                           Dimensional                                                                   change                                                              Porosity of                                                                             during     Flexural                                                                             Electrical                                        sintered  sintering  strength                                                                             resistivity                              EXAMPLE  body (%)  (%)        (MPa)  (Ω cm)                             ______________________________________                                        26       7.5       0.8        420    3 × 10.sup.-5                      27       15        0.8        340    3 × 10.sup.-5                      28       25        0.8        300    4 × 10.sup.-5                      29       30        0.8        250    6 × 10.sup.-5                      30       35        0.8        100    9 × 10.sup.-5                      ______________________________________                                    

EXAMPLES 31-35

The sintered body obtained in Example 1 was further subjected to hotpress sintering under 20 to 35,000 atm. and at 1,500° to 2,200° C. for150 minutes. The test results are shown in Table 4. As seen from thetable, reaction sintering in nitrogen under pressure notably decreasesthe porosity to less than 2% but only slightly increases dimensionalchange. Also, the TiN reacted sintered bodies showed a very lowelectrical resistivity of 3×10⁻⁵ Ωcm and a flexural strength of about500 to 650 MPa.

                  TABLE 4                                                         ______________________________________                                                                    Flexural                                                                              Electrical                                       HIP        Porosity  strength                                                                              resistivity                               Example                                                                              conditions (%)       (MPa)   (Ω cm)                              ______________________________________                                        31     N.sub.2,   1         580     3 × 10.sup.-5                              1800° C.,                                                              2000 atm.                                                              32     N.sub.2,   0         650     3 × 10.sup.-5                              2200° C.,                                                              1000 atm.                                                              33     N.sub.2,   1         576     3 × 10.sup.-5                              1500° C.,                                                              3500 atm.                                                              34     N.sub.2,   1         587     3 × 10.sup.-5                              1800° C.,                                                              850 atm.                                                               35     N.sub.2,   2         520     3 × 10.sup.-5                              2000° C.,                                                              20 atm.                                                                ______________________________________                                    

EXAMPLE 36

A sintered body was obtained by following the same procedure as Example1, except for change of the ratio of a metallic Ti powder having anaverage particle size of 1 μm and the ratio of SiC powder having anaverage particle size of 3 μm. The obtained sintered body was a 2-phasemixture in which the reaction product TiN was bound with SiC.

FIG. 6 shows the results of a test on the relation between electricalresistivity and blending ratio of material. It is seen that resistivityis variable within the range of 10⁻² to 10⁻⁵ Ωcm according to theblending ratio of material.

EXAMPLES 37-49

Molding and reaction-sintering of Example 1 were carried out with themixtures of metallic Ti powder having an average particle size of 1 μmand various types of powdered inorganic compounds shown in Table 6 toobtain the sintered bodies. These sintered bodies were the mixtures inwhich the reaction product TiN was bound with various types of inorganiccompounds.

It is noted that resistivity is variable according to the type andblending ratio of inorganic compound used.

EXAMPLE 50

A reaction sintered body was produced by the same process as Example 1,except for change of the amount of thermoplastic resin binder used. FIG.7 is a graph showing the relation between the amount of binder used andflexural strength. As seen from the graph, flexural strength sharplylowers when the amount of binder used is too small or too large. It isseen that use of a binder in an amount of 4 to 15% is desirable as aflexural strength of about 200 MPa or more can be obtained in this case,5 to 14% is especially recommended as a flexural strength of about 300MPa or above is obtainable.

                  TABLE 6                                                         ______________________________________                                                    Inorganic compound                                                                          Electrical                                                      used as starting                                                                            resistivity                                         Example     material (wt %)                                                                             (Ω cm)                                        ______________________________________                                        37          TiN(40)       3 × 10.sup.-5                                 38          Al.sub.2 O.sub.3 (60)                                                                       5 × 10.sup.-2                                 39          Si.sub.3 N.sub.4 (30)                                                                       4 × 10.sup.-4                                 40          Si.sub.3 N.sub.4 (50)                                                                       6 × 10.sup.-3                                 41          Si.sub.3 N.sub.4 (60)                                                                       9 × 10.sup.-2                                 42          Cr.sub.3 C.sub.2 30)                                                                        9 × 10.sup.-5                                 43          TiB.sub.2 (15)                                                                              6 × 10.sup.-5                                 44          TiSi.sub.2 (10)                                                                             5 × 10.sup.-5                                 45          TaN(40)       8 × 10.sup.-5                                 46          SiO.sub.2 (30)                                                                              6 × 10.sup.-4                                 47          Si.sub.2 N.sub.2 O(20)                                                                      4 × 10.sup.-4                                 48          VC(15)        7 × 10.sup.-5                                 ______________________________________                                    

EXAMPLE 51

A conductive ceramic made of a TiN sintered body obtained in Example 1was adapted to a current collector and a collector ring of an Acgenerator for automobiles, and the current collecting properties of saidelements were examined. The results are shown in Table 5. It is seenthat the products according to the present invention are less in wearand higher in wear resistance than the conventional copper-madecollector ring and carbon-made current collector, and also remain freeof any color change.

The test was conducted by revolving at a rate of 30,000 r.p.m. and acollector current density of 70 A/cm².

                  TABLE 5                                                         ______________________________________                                                                  Comp.                                                              Example 51 Example 2                                           ______________________________________                                        Collector ring TiN        Copper                                              Current collector                                                                            TiN        Carbon                                              Coefficient    0.13       0.20                                                of friction                                                                   State    current   No change  Had streaks                                     of       collector in luster                                                  abraded  Collector No change  Color changed into                              surface  ring      in luster  dark brown                                      Wear     Current   ≈0 0.82 μm                                               collector                                                                     Collector 0.5-1 μm                                                                              10-20 μm                                              ring                                                                 Sparks         None       None                                                ______________________________________                                    

EXAMPLE 52

50% by weight of metallic Si powder having an average particle size of0.9 μm was mixed with 50% by weight of Al₂ O₃ powder having an averageparticle size of 1 μm. Then a polyethylene type thermoplastic resin wasadded in an amount of 7 parts by weight to 100 parts by weight of thestarting material, and the mixture was kneaded by a press kneader underheating at 160° C. (softening point of the resin) for 4 hours. Aftercooling, the kneaded material was ground to a powder of less than 10mesh to prepare a material for insulator ceramic A.

Separately from the above, there was formed a mixture comprising 20% byweight of metallic Ti powder (average particle size: 1.6 μm) and 80% byweight of SiC powder (average particle size: 1 μm), then a polyethylenetype thermoplastic resin was added in an amount of 5 parts by weight to100 parts by weight of starting material, and the mixture was kneaded bya press kneader similarly under heating at 160° C. for 4 hours. Aftercooling, the kneaded material was ground to a powder of less than 10mesh to prepare a material for conductive ceramic B.

Then the materials of ceramic A and ceramic B were packed successivelyin a metal to make a cylindrical collector ring for motor shown in FIGS.8 and 9. The reaction sintering conditions were the substantially sameas in Example 1.

After removing the binder in the molding, the latter was heated in anitrogen atmosphere to obtain a ceramic complex. Dimensional change thatoccurred in forming a sintered body from the molding was as small as0.2%, and no cracks formed. Resistivity of ceramic A was about 10¹⁴ Ωcmand that of ceramic B was 7×10⁻⁵ Ωcm. Conductor and insulator werefirmly bound to each other with the reaction products Si₃ N₄ and TiN.

In the similar way, the ceramics with different electrical resistivitiescan be integrally molded and sintered in a laminar form by changing thetype and blending ratio of metallic powder and inorganic compound.

EXAMPLE 53

A commutator having the shape shown in FIG. 10 was produced in the sameway as described above by using ceramics A and B of Example 52. In thisexample, too, dimensional change was as small as 0.2%, and the producthad excellent wear resistance. In FIG. 10 (a) is a schematicalcross-sectional view of the commutator, and FIG. 10(b) is a schematicalside elevational view thereof.

EXAMPLE 54

A U-shaped heater (resistance: 0.1 Ω, resistivity: 6×10⁻³ Ωcm) such asshown in FIG. 11(a) and FIG. 11(b) was made by using electricallyconductive ceramic obtained in Example 40. A metallic layer is providedat terminal 1 of this heater. The heater terminal electricity V_(H),current V₁ and heater end temperature T_(H) at the time of applicationof a DC voltage of 12V to said heater terminal are shown in FIG. 12. Theheater end temperature reached 900° C. in about 0.8 second, 1,200° C. inabout 1.2 second and 1,500° C. in about 1.8 second, indicating thequickly heated property of the heater. On the other hand, load currentdecreased (indicating positive resistance temperature properties) as theheater temperature increased with electrification. This helps to preventtrouble such as fusing of the heater by thermal runaway.

EXAMPLE 55

An armature unit in an automobile starter such as illustrated in FIG. 13was manufactured in the manner described below, assembled in anautomobile and tested. In the drawing, numeral 3 denotes armature, 4yoke, 5 planetary reduction gear, 6 ring gear, 7 pinion, 8 bracket, 9magnetic switch, 10 permanent magnet, 11 coil conductor, 12 brush, and13 commutator.

A mixture was formed comprising 50% by weight of metallic Si powderhaving an average particle size of 0.9 μm and 50% by weight of Al₂ O₃powder having an average particle size of 1 μm. Then a polyethylene typethermoplastic resin was added in an amount of 7 parts by weight to 100parts by weight of the starting material and kneaded by a press kneaderat 160° C. for 4 hours. The kneaded mixture was ground to a powder ofless than 10 mesh to prepare a base material for insulator ceramic A.

Separately, a mixture comprising 80% by weight of metallic Ti powderhaving an average particle size of 1.6 μm and 20% by weight of TiNpowder having an average particle size of 1 μm was formed, followed byaddition of a polyethylene type thermoplastic resin in an amount of 5parts by weight to 100 parts by weight of the starting material andkneading by a press kneader at 160° C. for 4 hours The kneaded mixturewas ground to a powder of less than 10 mesh to prepare a base materialfor conductor ceramic B. Then said material for ceramic A and materialfor ceramic B were filled in a mold and molded into a product such asshown in FIG. 15. After removing the binder, the molding was heated in anitrogen atmosphere to obtain a ceramic complex. Only 0.2% ofdimensional change was seen in conversion from the molding into thesintered body, and no cracking took place. Resistivity of ceramic A wasabout 10¹⁴ Ω cm and that of ceramic B was 4×10⁻⁵ Ω cm. Both theconductor and insulator were securely bound with the reaction productsSi₃ N₄ and TiN.

The conventional copper wire was replaced by ceramic coil formed bycoating the copper wire with ceramic.

There could be produced a lightweight starter having excellent slidingcharacteristics and not becoming noncombustible even when the internaltemperature of the starter becomes close to 400° C. as the test resultsconfirmed.

EXAMPLE 56

A brush and slip ring in an alternator for automobiles shown in FIG. 14were made in the same manner as Example 55 so as to have a geometryshown in FIG. 15, and the product was incorporated in an automobile andtested. In FIG. 14, numeral 14 designates pulley, 15 fan, 16 frontbracket, 17 stator, 18 rotor, 19 brush, 20 slip ring, 21 silicon diode,and 22 rear bracket.

The test results are shown in Table 6. The test conditions were 40,000r.p.m. and 60 A/cm² as collector current density. According to thepresent invention, it is possible to obtain a lightweight alternatorhaving excellent wear resistance and spark control.

FIG. 16(a) shows a slip rig, and FIG. 15(b) shows a brush.

                  TABLE 6                                                         ______________________________________                                        Example 2                                                                     ______________________________________                                        Slip ring                   TiN                                               Brush                       TiN                                               Coefficient of              0.12                                              friction                                                                      State of        Brush       Lustrous                                          abraded         Slip ring   Lustrous                                          surface                                                                       Wear            Brush       ≈0                                                        Slip ring   0.8-1 μm                                       Sparks                      None                                              ______________________________________                                         Test conditions: 40,000 r.p.m.; collector current density = 60 A/cm.sup.2                                                                              

What is claimed is:
 1. A conductor material made of an electricallyconductive nitride produced by reaction sintering a shaped bodyconsisting essentially of (a) particles of at least one metal from Ti,Zr, V, Nb, Ta, Cr, W, Fe and Ni, said particles being free of Si, and(b) particles of at least one inorganic compound selected from acarbide, nitride, oxynitride, oxide silicide and boride in a CO-freenitrogen-containing atmosphere at a temperature below the melting pointof the mixed powder of (a) and (b), so that said particles of said atleast one inorganic compound are linked to another by particles andwhiskers of nitride of said at least one metal formed by said reactionsintering, said conductor material having an electrical resistivity of10¹⁴ to 10⁻⁵ Ω cm, wherein said conductor material has a porosity below30%.
 2. The conductor material made of an electrically conductivenitride according to claim 1 having an electrical resistivity at roomtemperature of 1×10⁻⁵ Ω cm to 5×10⁻⁵ Ω cm. PG,41
 3. A conductor materialmade of particles and whiskers of an electrically conductive nitrideproduced by reaction sintering a shaped body consisting essentially of(a) particles of at least one metal from Ti, Zr, V, Nb, Ta, Cr, W, Feand Ni, said particles being free of Si, and (b) particles of at leastone inorganic compound selected from carbide, nitride, oxynitride,oxide, silicide and boride in a CO-free nitrogen-containing atmosphereat a temperature below the melting point of the mixed powder of (a) and(b), said conductor material having an electrical resistivity of 10¹⁴ to10⁻⁵ Ω cm, wherein said conductor material has a porosity below 30%. 4.A conductor material made of an electrically conductive nitride producedby reaction sintering a shaped body consisting essentially of (a)particles of at least one metal from Ti, Zr, V, Nb, Ta, Cr, W, Fe andNi, (b) particles of at least one of silicon and aluminum, and (c)particles of at least one inorganic compound selected from a carbide,nitride, oxynitride, oxide, silicide and boride, wherein said shapedbody has a particle volume packing density of at least 60 vol % and saidparticles of at least one of silicon and aluminum are contained in anamount of 90% by volume or less, in a CO-free nitrogen-containingatmosphere at a temperature below the melting point of the mixed metalpowder of (a), (b) and (c), so that said particles of said at least oneinorganic compound are linked to one another by particles and whiskersof nitride of said at least one metal formed by said reaction sintering,said conductor material having an electrical resistivity of 10¹⁴ to 10⁻⁵Ω cm or less, wherein said conductor material has a porosity below 30%.5. The conductor material made of an electrically conductive nitrideaccording to claim 4, having an electrical resistivity at roomtemperature of 1×10⁻⁵ Ω cm to 5×10⁻⁵ Ω cm.
 6. A conductor material madeof particles and whiskers of an electrically conductive nitride producedby reaction sintering a shaped body consisting essentially of (a)particles of at least one metal from Ti, Zr, V, Nb, Ta, Cr, W, Fe andNi, (b) particles of at least one of silicon and aluminum, and (c)particles of at least one inorganic compound selected from a carbide,nitride, oxynitride, oxide, silicide and boride, wherein said shapedbody has a particle volume packing density of at least 60 vol % and saidparticles of at least one of silicon and aluminum are contained in anamount of 90% by volume or less, in a CO-free nitrogen-containingatmosphere at a temperature below the melting point of the mixed metalpowder of (a), (b) and (c), said conductor material having an electricalresistivity of 10¹⁴ to 10⁻⁵ Ω cm or less, wherein said conductormaterial has a porosity below 30%.
 7. A ceramic composite article whichcomprises a laminate of at least first and second layers, each of saidfirst and second layers being made of the reaction-sintered conductormaterials claimed in any one of claims 1, 3, 4 and 6, said first layerhaving a different electrical resistivity than said second layer, andsaid first and second layers being integrally sintered.
 8. The conductormaterial made of an electrically conductive nitride according to claim3, having an electrical resistivity at room temperature of 1×10⁻⁵ Ω cmto 5×10⁻⁵ Ω cm.
 9. The conductor material made of an electricallyconductive nitride according to claim 6, having an electricalresistivity at room temperature of 1×10⁻⁵ Ω cm to 5×10⁻⁵ Ω cm.
 10. Aconductor material according to claim 4, wherein said (a) particles ofat least one metal are selected from the group consisting of Ti, Zr, V,Nb, Ta, Cr, W and Ni.